The role of the inertia of cloud drops in the evolution of the spectra during drop growth by diffusion (original) (raw)
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Effects of in-cloud nucleation and turbulence on droplet spectrum formation in cumulus clouds
Quarterly Journal of the Royal Meteorological Society, 2002
Drop spectrum evolution is investigated using a moving mass grid microphysical cloud parcel model containing 2000 mass bins and allowing turbulent effects on droplet collisions. Utilization of precise methods of diffusion and collision drop growth eliminates any arti cial droplet spectrum broadening. Simulation of continental, intermediate and maritime clouds is conducted using different concentrations of cloud condensation nuclei and different vertical velocities at the cloud base.
Quarterly Journal of the Royal Meteorological Society, 1997
The mechanisms of drop-concentration inhomogeneity formation are studied using both a numerical simulation with a model of isotropic and homogeneous turbulence, and analytical methods. It is shown that atmospheric turbulence can create a significant drop-concentration inhomogeneity due to the effects of drop inertia. %o types of area in the turbulent flow are revealed. Drops tend to leave the areas of 'drop vortices' and collect within the zones out of the vortices. As a result, the zones of drop-track collection turn out to be the zones of enhanced drop concentration. The rate of concentration enhancement is studied for drops of different sizes using the Monte Carlo method. It is shown that the drop flux velocity divergence and droplet-concentration variations reach their maximum at 100 p m drop radius. Zones of enhanced drop concentrations are stretched along the drop tracks. Characteristic scales of drop-concentration fluctuations along the drop tracks are of the order of several metres or even a few tens of metres. Across the drop tracks, the characteristic scale of concentration pulsations is of the order of a few centimetres.
Turbulence effects on droplet growth and size distribution in clouds—A review
Journal of Aerosol Science, 1997
The paper is focused on inertia effects among drops moving within a turbulent cloud on size distribution evolution and formation of rain. Two related mechanisms are discussed: (1) the occurrence of inertia-induced relative velocities between drops falling within a turbulent flow, and (2) the tendency of inertial drops to concentrate within certain areas of turbulent flow with a corresponding concentration decrease elsewhere. It is shown that these turbulence-induced mechanisms lead to a broadening of the droplet spectrum during the early stages of cloud formation. Turbulence also decreases the minimum size of droplets are able to collide with the smaller ones. Thus, turbulence seems to bridge drop growth by condensation and coagulation. Due to the non-linear nature of the kinetic equation of coalescence, effects of positive fluctuations of drop concentration dominate and lead to a faster droplet spectrum broadening. Turbulence effects for ice particles, especially ice crystals and snowflakes, are expected to be much more pronounced than those demonstrated for water drops, because of a smaller terminal fall velocity of ice particles and a comparably large mass. Large turbulent eddies can concentrate ice crystals within certain areas, where ice crystals concentration will be substantially greater than mean ice crystal concentration. Thus, turbulence can contribute to the formation of high ice crystal concentration observed in mixed-phase clouds. The investigation of turbulence effects in clouds drop evolution is a field currently undergoing very rapid development. Some unsolved problems are discussed.
Broadening of droplet size distributions from entrainment and mixing in a cumulus cloud
Quarterly Journal of the Royal Meteorological Society, 2005
Quantitative predictions of the relationship between the droplet size-distribution width and entrainment in warm cumulus have been elusive, largely because of the difficulty in representing the extent of the scales involved. A new modelling framework is presented as a first step toward quantitative predictions of droplet size distributions resulting from entrainment, consisting of a three-dimensional cloud model coupled with a Lagrangian microphysical parcel model. The cloud model represents turbulent cloud dynamics but parametrizes microphysical processes such as condensation, and the parcel model complements this approach by performing explicit microphysical calculations within the kinematic and thermodynamic constraints established by the cloud model. The parcel model is run along trajectories all ending at the same point in the cloud, and the individual droplet size distributions are averaged together at this point to represent the turbulent mixing together of the droplets produced by these different parcel trajectories. The results replicate some important features of observed cloud droplet size distributions, including large widths, the continued presence of small droplets high in the clouds, and the bimodal structure. The origin of these features in these calculations is the variability introduced by entrainment, which leads to possibilities for droplets to encounter varying supersaturation histories during their transit through the cloud to the point of observation. Droplet sizes larger than those calculated for adiabatic ascent are also produced, with possible implications for coalescence initiation.
Effects of entrainment and mixing on droplet size distributions in warm cumulus clouds
Journal of Advances in Modeling Earth Systems, 2014
A long-standing problem in cloud physics is the broadening of the cloud droplet spectrum in warm cumulus clouds. To isolate the changes of the droplet size distribution (DSD) due to entrainment and turbulent mixing, we used the Explicit Mixing Parcel Model (EMPM). The EMPM explicitly represents spatial variability due to entrainment and turbulent mixing down to the smallest turbulence scales in a onedimensional domain. Several thousand individual droplets evolve by condensation or evaporation according to their local environments. We used EMPM results to characterize the evolution of the DSD due to entrainment and isobaric mixing for a wide range of conditions in a 20 m domain, including variations in entrained environmental air fraction, the turbulence dissipation rate, the size of the entrained blobs, and the relative humidity of the entrained air. We found that the broadening of the DSD due to entrainment and isobaric mixing for a specific value of the entrained air relative humidity depends only on the eddy mixing time scale and the LWC after mixing. Broadening increases substantially as the evaporation time scale decreases due to decreasing relative humidity of the entrained air. Our results also show that it is possible to parameterize the effects of entrainment and mixing on the droplet number concentration. The comprehensive results obtained for one set of values of entrained air relative humidity, droplet size, and droplet concentration should be extended to other values.
[1] Cloud droplet spectral width (standard deviation; sigma; s) was inversely related to cloud condensation nuclei (CCN) concentrations (N CCN) in the Rain in Cumulus over the Ocean (RICO) project. This was determined from thorough comparisons between flight-averaged N CCN and microphysics of the lowest altitude cloud passes of 17 RICO flights. Adiabatic model predictions of droplet spectra based on complete below cloud CCN spectra and within-cloud vertical velocity (W) showed good agreement with measured droplet concentrations and mean diameter (MD) but s predictions were only weakly correlated with measured s. Significantly better s predictions were obtained for mixtures of droplet spectra for distributions of W that were measured in each flight. Adiabatic model predictions for various W applied uniformly to all RICO flights displayed a trend of correlation coefficients (R) for s-N CCN plotted against W that changed from positive to negative with increasing W. The s-N CCN positive R range at low W corresponds to previous results in stratus clouds where it has been suggested that albedo calculations that include s reduce the indirect aerosol effect (IAE). The s-N CCN negative R range of higher W corresponds to more convective clouds such as RICO where albedo calculations that include s thus might seem to augment IAE.
Droplet growth in warm turbulent clouds
Quarterly Journal of the Royal Meteorological Society, 2012
In this survey we consider the impact of turbulence on cloud formation from the cloud scale to the droplet scale. We assess progress in understanding the effect of turbulence on the condensational and collisional growth of droplets and the effect of entrainment and mixing on the droplet spectrum. The increasing power of computers and better experimental and observational techniques allow for a much more detailed study of these processes than was hitherto possible. However, much of the research necessarily remains idealized and we argue that it is those studies which include such fundamental characteristics of clouds as droplet sedimentation and latent heating that are most relevant to clouds. Nevertheless, the large body of research over the last decade is beginning to allow tentative conclusions to be made. For example, it is unlikely that small‐scale turbulent eddies (i.e. not the energy‐containing eddies) alone are responsible for broadening the droplet size spectrum during the i...
Evidence for inertial droplet clustering in weakly turbulent clouds
Tellus B, 2007
A B S T R A C T Simultaneous observations of cloud droplet spatial statistics, cloud droplet size distribution and cloud turbulence were made during several cloud passages, including cumulus clouds and a stratus cloud. They provide evidence that inertial droplet clustering occurs even in weakly turbulent clouds. The measurements were made from the Airborne Cloud Turbulence Observation System suspended from a tethered balloon. For a profile through a stratus cloud with gradually changing droplet Stokes number, droplet clustering, quantified by the pair correlation function, is observed to be positively correlated with the droplet Stokes number. This implies that the droplet collision rate, which is relevant to drizzle formation via droplet coalescence, depends not only on the droplet size distribution, but also on the cloud turbulence. For cumulus clouds, the relation between droplet clustering and Stokes number seems more complicated. Stokes number is determined by measuring droplet size and local energy dissipation rate, the latter requiring highresolution air velocity measurements not possible on fast-flying aircraft.
Journal of Applied Meteorology, 2001
A new feature of cloud structure has been discovered while analyzing the measurements obtained in situ in 57 clouds by the Fast Forward-Scattering Spectrometer Probe (FSSP). By means of a novel technique of statistical analysis, it is shown that droplets form distinct ''communities'' of about 1-cm scale that differ in concentration, thus creating a highly inhomogeneous cloud microstructure (inch clouds). Those droplet clusters can be found all over the cloud volume and appear to be induced by droplet inertia within a turbulent flow. An increase in turbulence intensity and droplet inertia results in an increase of concentration fluctuations. The authors believe that this finding is the first direct evidence of turbulence-inertia impact on droplet motion in clouds that leads to formation of microstructure conductive to precipitation formation.
Growth of Cloud Droplets in a Turbulent Environment
Annual Review of Fluid Mechanics, 2013
Motivated by the need to resolve the condensation-coalescence bottleneck in warm rain formation, a significant number of studies have emerged in the past 15 years concerning the growth of cloud droplets by water-vapor diffusion and by collision-coalescence in a turbulent environment. With regard to condensation, recent studies suggest that small-scale turbulence alone does not produce a significant broadening of the cloud-droplet spectrum because of the smearing of droplet-scale fluctuations by rapid turbulent and gravitational mixing. However, different diffusional-growth histories associated with large-eddy hopping could lead to a significant spectral broadening. In contrast, small-scale turbulence in cumulus clouds makes a significant contribution to the collision-coalescence of droplets, enhancing the collection kernel up to a factor of 5, especially for droplet pairs with a low gravitational collision rate. This moderate level of enhancement has a significant impact on warm rain initiation. The multiscale nature of turbulent cloud microphysical processes and open research issues are delineated throughout.